Handle Booleans in Conditional Styles

[oleibman]

You can set up a conditional style to, say, apply to cells equal to boolean values. For such conditions, the Excel XML specifies TRUE or FALSE. It is noteworthy that false matches empty cells as well as FALSE, but not 0; similarly TRUE does not match 1. The Xlsx Writer just casts these values to string, which will not work properly. The Xlsx Reader treats the values as strings, so it won't work properly either. This PR corrects both. Also the doc blocks in Style/Conditional allow bool in some places, but not in others; these are corrected but no executable code is changed there.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

[MarkBaker]

I'm longing for the day when we've reached a minimum level of PHP that we can do:

$objConditional->addCondition(
    match((string) $formulax) {
        'TRUE' => true,
        'FALSE' => false,
        default => $formulax
    }
);

So much easier to read than if/elseif/else.

Does this need to make allowance for locale variants in TRUE/FALSE strings (VRAI/FAUX or VERO/FALSE)? Ah! No; this is in the Reader, so it'll be in English